Radar Sounding Research Articles

Delay Doppler SAR Focusing and Quantitative Quality Control of the Radar for Europa Assessment and Sounding: Ocean to Near-Surface (REASON) Sounding Data Product

Synthetic aperture radar (SAR) focusing will be a fundamental step in the analysis of the radar sounding datasets collected by the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) instrument as part of NASA's upcoming Europa Clipper mission. Due to the flyby trajectory of the mission, REASON data acquisition will be distinct compared to other space-borne radar sounders, and therefore, require a tailored SAR focusing strategy. Here, we present a SAR focusing architecture based on the delay Doppler approach employed in US SHAllow RADar (SHARAD) data analysis with the following modifications for REASON data idiosyncrasies; an interpolation to a constant ground track interval to account for REASON's variable PRF; and an adaptive Doppler centroid estimation to account for the flyby geometry. The ability of our modified delay Doppler SAR focusing approach to focus space-borne datasets as well as its specific feasibility for REASON are demonstrated using both SHARAD and MARSIS datasets. In addition, we present a quantitative quality control framework based on pixel power probabilities and demonstrate how it can be leveraged to quantify the effects of SAR focusing and differentiate focused results generated with different processing parameters. Finally, we revisit and discuss the assumption of depth-independent SAR focusing implicit in the choice to construct a REASON SAR focusing approach based on the delay Doppler method and provide a comparison with depth-dependent SAR focusing for a simplified acquisition geometry.

Application of radar sounding of subsurface studies in the assessment of freight car descents

Application of radar sounding of subsurface studies in the assessment of freight car descents

СТРОЕНИЕ СНЕЖНО-ЛЕДОВЫХ ПЕРЕМЫЧЕК ПРОРЫВНЫХ ОЗЁР ПОЛУОСТРОВА БРОКНЕС (ОАЗИС ХОЛМЫ ЛАРСЕМАНН, ВОСТОЧНАЯ АНТАРКТИДА) ПО ДАННЫМ ГЕОРАДИОЛОКАЦИИ

During the summer field season of the 65 th Russian Antarctic Expedition a research aimed at studying the structure of the snow-­ice dams of the Lakes Progress and Discussion (Larsemann Hills, East Antarctica), which are characterized with annual outburst floods, was carried out. Survey was performed using ground­penetrating radar sounding complemented with non­core drilling and analysis of the aerial photo data acquired with unmanned aerial vehicle during the last field seasons. The results show that location of the waterways, which occur during the outbursts of the both lakes, does not change significantly year in year out and fits a linear depression in basement topography under the dam and a following flexure of the ice layer. During the winter period, the opened channels are being filled with snow, and thereby a natural soſtened zone is being formed. Further outburst flood propagates mainly within this zone. Monitoring survey of the snow­i-ce dam of the Progress Lake during the summer period showed that destruction of the dam does not happen rapidly when the outburst takes place, but begins a few weeks before it with gradual filtration within the snow layer.

An Integrated Technology of Ionospheric Backscatter Detection and Oblique Detection

Traditional single ground-based ionospheric detection methods often fail to obtain accurately ionospheric parameter information due to the measurement errors of detection systems and inverse algorithms. With the development of high frequency communication and radar sounding techniques, the ionospheric combined detection technology with multi-means is the most effective way to obtain the more accurate ionospheric characteristics. In this study, combined with ionospheric backscatter detection and oblique detection methods, an integrated ionospheric detection technology (quasi-backscatter detection technology) is proposed. The ionospheric parameters are obtained based on the minimum mean square error criterion and global searching method. The experimental results show that the inversion accuracy of ionospheric parameters is significantly improved by using quasi-backscatter detection system(42.3°N, GeoM). In addition, we simulate the propagation characteristics of the quasi-backscatter detection system by using ray tracing technique. And the variation characteristics between the propagation mode (and the propagation energy) and the angle of the transmitting-receiving beam are obtained.

A Spaceborne Multistatic Radar Sounding System for the Tomographic Observation of Polar Ice Sheets

Radar sounding plays an irreplaceable role in polar ice sheets research. Over the past 60 years, vehicle-borne and airborne systems have provided large amounts of topographic data on ice sheets. However, due to the limitations of atrocious weather in the polar regions and the platform operating distance, a large blind zone of observations still remains. Spaceborne radar systems can realize efficient observations of the Earth’s surface due to the wide swath of satellites and the penetration of clouds and rain by microwaves. However, existing remote sensing satellites still cannot observe ice beds, which are subject to severe radio attenuation and complex signal propagation in ice. In this article, a spaceborne multistatic radar sounding system named the BingSat-Tomographic Observation of Polar Ice Sheets (TOPISs) is proposed to achieve high resolution and stereoscopic observation. Over the polar regions, the satellite formation design via passive CubeSats with dipole antennas forms a large cross-track baseline. Based on that, cross-track resolution improvement, ice attenuation compensation, and surface clutter suppression are realized simultaneously. Single-input and multiple-output mode and MirrorSAR technology are employed to reduce the manufacturing cost and realize synchronization. With a high transmitting power and antenna gain, BingSat-TOPIS can penetrate several kilometers of ice sheets.

Internal layering structure and subglacial conditions along a traverse line from Zhongshan Station to Dome A, East Antarctica, revealed by ground-based radar sounding

Internal layering structure and subglacial conditions along a traverse line from Zhongshan Station to Dome A, East Antarctica, revealed by ground-based radar sounding

Martian roughness analogues of Europan terrains for radar sounder investigations

Understanding the implications of Europa's roughness on backscattered radar signals is central to ensure that the future radar sounding instruments heading for the Jovian icy moons, the Radar for Icy Moons Exploration (RIME) and the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON), will yield fruitful observations over their Europan targets. In this paper we analyse the roughness of Mars in terms of piecewise monofractal behaviour, and compare the derived Allan profiles to those of different types of Europan terrains. Using the MOLA dataset, we created roughness analogue maps for 12 different types of Europan terrains, and performed additional analysis with High Resolution Imaging Science Experiment (HiRISE) DTM data to complement our study. We found that Aeolis Planum, the flanks of Tharsis Montes, and the chaos terrain south of Olympus Mons are particularly rich in Europan roughness analogues. To put our findings into context, we present optical image comparisons between some Europan terrains and a given Martian roughness analogue, as well as SHARAD and MARSIS tracks passing over some of these areas of interest.

Nadir Detection of Lunar Lava Tube by Kaguya Lunar Radar Sounder

This article intends to establish a reasonable determination criterion for nadir detection of lunar lava tube by Kaguya Lunar Radar Sounder observation. We applied radar cross section analysis to a simple lava tube model that comprises vacuum/basalt media with flat media boundaries, which illustrated that radar echoes from a lava tube are continuously detected through a number of consecutive observations along orbit if the lava tube returns radar echoes strong enough. “Consecutive detection” makes the minimal determination criterion for the detection of a lava tube. We further investigated influence of curved ceiling of a lava tube on its detection using 2-D finite difference time domain (2-D FD-TD) simulation and radar cross section analysis with Geometrical Optics. The work revealed that basalt roof of a lava tube plays a role of a concave lens in the propagation process of a radar pulse hence contributes to reduce lava tube echo intensity in comparison to that of the simple lava tube model. The curved ceiling, however, does not affect the above derived determination criterion. Ambiguity test plays the crucial role in the detection of a lava tube by distinguishing surface clutter. We demonstrated three types of ambiguity tests, i.e., 1) multiorbit data analysis test, 2) surface scattering simulation test, and 3) surface slope echo analysis test. The last two tests were demonstrated on those data which had been documented as radar echoes from large scale lava tubes in Marius Hills in a recent literature. The test results indicated that they are nothing more than a collection of surface clutter.

An Estimation of Human‐Error Contributions to Historical Ionospheric Data

AbstractGround‐based radar sounders are used to characterize the dynamics and chemistry of Earth's upper atmosphere by using measurements of ionospheric peak electron density (NmF2) and its associated altitude (hmF2). Continuous sounder observations of the E and F regions of the ionosphere have been carried out regularly at dozens of stations worldwide since the midtwentieth century. A deep understanding of short‐ and long‐term upper atmospheric variability depends on a fundamental understanding of these observational data. The manual analysis of historical analog (predigital age) ionograms to derive the plasma frequency profiles and the ionospheric parameters hmF2 and NmF2 is a tedious procedure and susceptible to human error. In order to better understand this human error, a study is conducted in which ionograms from vertical sounders are manually scaled by a team of ionospheric researchers. The results of the study are then used to estimate the variability of the hand‐scaled ionospheric parameters foF2 and foE. Those results are then used to estimate the downstream impact on ionospheric models that use foF2 and foE as input. The results demonstrate that there can be large variability in the manual scaling of foF2 and fmaxE from vertical incidence ionograms. However, the participants did typically better than 5% uncertainty for benign ionograms. A long‐term analysis of hmF2 modeling exhibits low sensitivity to statistical errors imposed on foF2 and foE, but a short‐term analysis showed that modeled hmF2, neutral winds, and electron densities can be very different when small adjustments are made to foF2 and fmaxE.

Influence of the Estimation Accuracy of the Frequency Characteristics of Radar Targets on the Results of the Geometric Shape Identification of Local Reflectors

This article discusses the issues of introducing broadband technologies to civil and special-purpose radar systems. The popularity of these technologies in terms of solving numerous practical problems determines the relevance of research related to a detailed analysis of long-range portraits of objects observed at a new qualitative level. Using the results of broadband radar sounding, this study analyzes the influence of the systematic error of the measuring channel on the reliability of the identification of the geometric shape of the local reflectors of observation objects. A broadband radar-measuring channel with a linear frequency modulation of the signal is created on the basis of the generalized heterodyning method. Then, a technique for compensating systematic measurement errors for a given type of channel is proposed using calibration spheres. The systematic error in measuring the frequency characteristics (FCs) of the sphere is estimated, and an error indicator for estimating the FCs of objects is proposed. Furthermore, the dependence of the statistical indicators of the identification results of the geometric shape of local reflectors on the specified indicator is presented. The possibility of identifying the geometric shape of local reflectors through the parameters of their FCs is confirmed experimentally. The results obtained can be used to assess the reliability of the identification of the geometric shape of local reflectors at the calibration stage of the measuring channel and to make a decision on the feasibility of implementing algorithms for the identification of the geometric shape of local reflectors on an existing radiolocation station.

Analysis of Temporal and Structural Characteristics of Jovian Radio Emissions for Passive Radar Sounding of Jupiter’s Icy Moons

Recent studies have proposed that Jovian decametric radiation (DAM) can be effectively exploited for probing the subsurface of Jupiter’s icy moons by using passive radio sounding. However, these studies were based on the assumption that Jovian noise is white and stationary. Therefore, additional investigations into the temporal stability, predictability, and spectral properties of Jovian noise are required to fully assess the potential of passive radar sounding and improve the acquisitions planning. In this article, we investigate these properties of the Jovian DAM to understand their impact on radar sounding performance. This is done by analyzing the recently available radio spectra acquired by the JUNO Waves instrument. Results are also evaluated for the specific case of the Radar for Icy Moon Exploration (RIME) and the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) that have been selected for European Space Agency (ESA) and NASA missions to Ganymede and Europa. Our results show that the Jovian DAM is not perfectly white, but no severe distortions in the range response should be expected. The results on spatiotemporal occurrence show that Jupiter’s DAM has a variable probability of occurrence, which is rather sporadic for some frequency ranges. The results on RIME and REASON flybys show that the Jovian DAM occurrence probability is relatively low for selected sub-Jovian flybys at Europa. For the RIME Ganymede orbital phase, a large number of high occurrence passive acquisition opportunities are expected. The experimental results also show that a large bandwidth receiver would enhance the chance of recording Jovian noise.

Estimation of bulk permittivity of the Moon\u2019s surface using Lunar Radar Sounder on-board Selenological and Engineering Explorer

Site-dependent bulk permittivities of the lunar uppermost media with thicknesses of tens to hundreds meters were estimated based on the data from Lunar Radar Sounder onboard the Selenological and Engineering Explorer (SELENE). It succeeded in sounding almost all over the Moon’s surface in a frequency range around 5 MHz to detect subsurface reflectors beneath several lunar maria. However, it is necessary to estimate the permittivity of the surface regolith of the Moon in order to determine the actual depths to those reflectors instead of apparent depths assuming a speed of light in the vacuum. In this study, we determined site-dependent bulk permittivities by two-layer models consisting of a surface regolith layer over a half-space with uniform, but different physical properties from the layer above. Those models consider the electrical conductivity as well as the permittivity, whose trade-off was resolved by utilizing the correlation between iron–titanium content and measured physical properties of lunar rock samples. Distribution of the iron–titanium content on the Moon’s surface had already been derived by spectroscopic observation from SELENE as well. Four lunar maria, Mare Serenitatis, Oceanus Procellarum, Mare Imbrium, and Mare Crisium, were selected as regions of evident reflectors, where we estimated the following four physical properties of each layer, i.e., bulk permittivity, porosity, loss tangent and electrical conductivity to conclude the actual depths of the reflectors are approximately 200 m on average. The bulk permittivity ranges from 2.96 at Mare Imbrium to 6.37 at Oceanus Procellarum, whereas the porosity takes the values between 1.8 and 41.1% in the respective maria. It was found that although the bulk permittivity of the four lunar maria differs from a mare to a mare, it shows a good correlation with their composition, viz., their iron–titanium content.

Firn Clutter Constraints on the Design and Performance of Orbital Radar Ice Sounders

Radar sounding is a powerful tool for constraining subglacial conditions, which influence the mass balance of polar ice sheets and their contributions to global sea-level rise. A satellite-based radar sounder, such as those successfully demonstrated at Mars, would offer unprecedented spatial and temporal coverage of the subsurface. However, airborne sounding studies suggest that poorly constrained radar scattering in polar firn may produce performance-limiting clutter for terrestrial orbital sounders. We develop glaciologically constrained electromagnetic models of radar interactions in firn, test them against in situ data and multifrequency airborne radar observations, and apply the only model we find to be consistent with observation to assess the implications of firn clutter for orbital sounder system design. Our results show that in the very high-frequency (VHF) and ultrahigh-frequency (UHF) bands, radar interactions in the firn are dominated by quasi-specular reflections at the interfaces between layers of different densities and that off-nadir backscatter is likely the result of small-scale roughness in the subsurface density profiles. As a result, high frequency (HF) or low VHF center frequencies offer a significant advantage in near-surface clutter suppression compared to the UHF band. However, the noise power is the dominant constraint in all bands, so the near-surface clutter primarily constrains the extent to which the transmit power, pulselength, or antenna gain can be engineered to improve the signal-to-noise ratio. Our analysis suggests that the deep interior of terrestrial ice sheets is a difficult target for orbital sounding, which may require optimizations in azimuth processing and cross-track clutter suppression which complement existing requirements for sounding at the margins.

Radar Sounding of Subsurface Structure in Eastern Coprates and Capri Chasmata, Mars

AbstractWe surveyed the subsurface structure in eastern Coprates and Capri Chasmata in the equatorial region using high‐resolution visible images, digital terrain models, and radar sounding data. We identified subsurface reflectors in four areas of the chasmata. At the stratigraphic exposure on the chasmata walls, the corresponding depth of the reflector is ∼60 m. The bulk dielectric constants of the layers above the reflectors are calculated as 3.4–4.0, suggesting a rock‐air mixture with ∼39.3% and 46.1% porosity or a rock‐air‐ice mixture with <21.2% water ice fraction. This high porosity corresponds to nonwelded and unconsolidated sediments emplaced by aeolian, fluvial, and volcanic activities. If water ice actually exists, further studies and discussions are required for the mechanism to maintain it within low latitudes.

The Global Search for Liquid Water on Mars from Orbit: Current and Future Perspectives.

Due to its significance in astrobiology, assessing the amount and state of liquid water present on Mars today has become one of the drivers of its exploration. Subglacial water was identified by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard the European Space Agency spacecraft Mars Express through the analysis of echoes, coming from a depth of about 1.5 km, which were stronger than surface echoes. The cause of this anomalous characteristic is the high relative permittivity of water-bearing materials, resulting in a high reflection coefficient. A determining factor in the occurrence of such strong echoes is the low attenuation of the MARSIS radar pulse in cold water ice, the main constituent of the Martian polar caps. The present analysis clarifies that the conditions causing exceptionally strong subsurface echoes occur solely in the Martian polar caps, and that the detection of subsurface water under a predominantly rocky surface layer using radar sounding will require thorough electromagnetic modeling, complicated by the lack of knowledge of many subsurface physical parameters. Higher-frequency radar sounders such as SHARAD cannot penetrate deep enough to detect basal echoes over the thickest part of the polar caps. Alternative methods such as rover-borne Ground Penetrating Radar and time-domain electromagnetic sounding are not capable of providing global coverage. MARSIS observations over the Martian polar caps have been limited by the need to downlink data before on-board processing, but their number will increase in coming years. The Chinese mission to Mars that is to be launched in 2020, Tianwen-1, will carry a subsurface sounding radar operating at frequencies that are close to those of MARSIS, and the expected signal-to-noise ratio of subsurface detection will likely be sufficient for identifying anomalously bright subsurface reflectors. The search for subsurface water through radar sounding is thus far from being concluded.

Mars Surface Imaging by Exploiting Off-Nadir Radar Sounding Data

Radar sounder surface imaging is a rather unexplored approach to the analysis of planetary bodies. While a radar sounder is an instrument specifically designed for subsurface investigations, a particular set of power measurements (denoted as off-nadir surface echoes) can be exploited together with an external digital elevation model to produce images of the investigated surface at meters wavelength. The use of the off-nadir data may also reveal the presence of previously undetected subsurface features. In this article, we present a method for producing surface roughness images by high-frequency (HF) radar sounder data. The study of surface roughness in the HF band is particularly useful for both geologic studies and landing-zone reconnaissance as it is evaluated at meters to hundreds of meters horizontal scale. The proposed method combines off-nadir data of the Shallow Radar Sounder (SHARAD) with the Mars Orbiter Laser Altimeter (MOLA) digital elevation model. The produced roughness images at 20 MHz (15-m wavelength) of the Martian surface provide higher coverage and resolution of the surface roughness characterization at a 10-100-m horizontal scale than previous SHARAD work. By comparing the experimental roughness image with the one obtained by radar backscattering simulations, it is possible to identify subsurface features. In our experiments, we were able to produce a bidimensional image of a previously undetected large buried crater (10 km ×12 km) located in the Nili Fossae. This finding opens up new possibilities in exploiting radar sounding data for better detecting shallow subsurface features.

Permanent Scatterers in Repeat-Pass Airborne VHF Radar Sounder for Layer-Velocity Estimation

Englacial layer velocity can provide insights on the vertical-velocity structure of the ice sheets. We present a repeat-pass interferometric approach that allows the estimation of the vertical englacial layer velocity using the radar sounder data. In contrast to the ground-based sensors, the airborne radar sounder data can potentially be used to estimate the layer velocity on a continental scale. When merged with the horizontal surface velocity and the numerical models, layer velocity can support the 3-D analysis of the ice flow and structure. Our aim is to provide the proof-of-concept demonstration that, similar to the side-looking synthetic aperture radar for imaging, the airborne radar sounder data can be used to estimate the subwavelength displacement of the englacial radio-stratigraphic layers. To achieve this, we use the phase and magnitude acquired repeatedly over the same region of interest. After the extraction of the crossing points, two acquisitions are finely registered. Then, we compute the interferometric phase for the englacial layers (which are shown to behave as PSs in the nadir sounding geometry), to estimate a vertical displacement and a velocity profile. We present the results over East Antarctica using data from the high-capability radar sounder (HiCARS) system. We show two scenarios that demonstrate the feasibility, limitations, and requirements of this approach.

Radar Sounding Confirms a Hydrologically Active Deep-Water Subglacial Lake in East Antarctica

Lake CookE2, upstream of Cook Glacier in East Antarctica is an ‘active’ subglacial lake, which experiences episodic discharge and recharge of basal water. Although around 130 active lakes are known to exist, the majority are not able to be identified by radar sounding techniques, suggesting they are ephemeral and/or distributed stores of small amounts of water rather than permanent significant singular features. Airborne ice-sounding radar data from Lake CookE2, a known ‘active’ subglacial lake, reveal a bright and flat ice-bed interface, providing clear evidence of deep (>10 m) water surrounded by elevated topography, however. The data reveal the lake area is ~46 km2; 3 times less than a previous estimated (145 km2) from ICESat surface altimetry, indicating a possible bias in identifying subglacial lake area from surface depressions. Using time-series altimetry from ICESat, Cryosat-2 data, and the Reference Elevation Model of Antarctica, we re-estimate the lake discharged ~2.93 km3 of water (or ~63.8 m in lake level) between February 2006 and January 2011. Subsequently, the ice surface over the lake rose steadily and experienced a mean uplift of ~11 m between January 2011 and November 2016, indicating continuous recharge with total volume increase of ~0.51 km3. The lake is recharging at a rate of ~1.33 m/year, which means it will take another ~39 years to reach the lake level that triggered the previous discharge.

A 2-D Pseudospectral Time-Domain (PSTD) Simulator for Large-Scale Electromagnetic Scattering and Radar Sounding Applications

This article discusses the implementation of a 2-D pseudospectral time-domain (PSTD) full-wave simulator for solving large-scale low-frequency (e.g., HF) electromagnetic (EM) scattering problems with the application of radar sounding of planetary subsurfaces. Compared to other computational EM algorithms, the PSTD solver is both memory-efficient and accurate for sounding applications. New domain designs are developed to efficiently simulate 2-D scattering of half-space media for normal and oblique incidence from arbitrary wave sources. As a validation of the PSTD simulator, the simulated 2-D scattering radar cross width (RCW) is compared with the analytical solutions of both point targets (dielectric cylinders) and distributed targets (random rough surfaces), for the first time, where the frequency and angular (bistatic scattering) dependence are studied with various choices of grid sampling resolution. Furthermore, the PSTD solver is applied to passive synthetic aperture radar (SAR) sounding problems (single transmitter and several receivers), for the first time, where various scenarios (e.g., cylinder, surface, and volume) are demonstrated and the targets are correctly resolved after focusing, indicating an accurate simulation of the phase history. Finally, an example of using the solver is shown for emulating 3-D large-scale radar sounding problems with cross-track surface and subsurface scattering. This is particularly useful to simulate radar sounding returns and SAR-focused imagery of large-scale subsurface structures to better support planetary missions with radar sounding instruments.

Структура автоматизованої системи управління інформаційною стійкістю наземної оглядової радіолокаційної станції в умовах активних завад

Надано інформаційний опис структури системи управління для пошуку і реалізації на різних рівнях та етапах управління закону управління інформаційною стійкістю режимів зондування РЛС при загрозах і впливах активних завад прикриття і самоприкриття, який узагальнює результати комплексного аналізу впливу на її побудову ряду системо утворюючих чинників. При цьому реалізується повна функція ситуаційного управління із застосуванням різних форм управління для когнітивного пошуку обґрунтованого і оперативного управлінського рішення з урахуванням поставлених цілей і ресурсних обмежень. Вона передбачає цілеспрямоване проведення на вході і виході режимів зондування кожної проблемної області аналіз і прогноз розвитку станів і динамік складових і в цілому конфліктної ситуації, а також умов прийняття рішень. Розглянута багатоконтурна структура управління дозволяє раціонально використовувати ресурсні можливості РЛС щодо забезпечення завадозахищеності її режимів зондування проблемних областей зони огляду і компонент їх систем управління з урахуванням традиційних і нових інформаційних загроз стратегії РЕП.